1. Field of the Invention
The present invention relates to an oxygen generating electrode for use in the field of electrochemical industry and a method of producing the same, and more particularly, to an insoluble electrode for oxygen generation and a method of producing the same, which electrode has excellent durability when used as an anode for electrolytic processes such as electroplating, electrolytic refining, electrolytic synthesis of organic materials, and protection of cathodes against corrosion.
2. Description of the Related Art
When plating a base material made of, for example, iron, copper, titanium or stainless steel, with zinc, copper or chromium by electrolytic process, the base material is immersed in a plating bath containing zinc sulfate, copper sulfate or chromium sulfate as a main component, with optional addition of sulfuric acid. Besides the base material serving as a cathode, an oxygen generating electrode, which serves as an anode, is immersed in the plating bath so that electrolytic reaction takes place when an electric current is passed between the electrodes.
For the oxygen generating electrode, a lead electrode is conventionally used.
Lead electrodes can be manufactured at low cost, and their solubility to sulfuric acid is low even though the lead dissolves in the plating bath. Accordingly, the concentration of lead in the plating bath can be maintained at a low level, and the plated surface of the base material is less influenced by lead ions.
The lead electrode, however, has a problem when used as the oxygen generating electrode. Namely, the overvoltage during the oxygen generation becomes high, and the distance between the electrodes increases because the lead dissolves as electrolysis progresses, increasing the electrolytic voltage of the bath as a whole. Thus, it is difficult to cut down the power consumption during electrolysis, and adjustment of the interelectrode distance and replacement of electrodes are frequently required since the thickness of the electrode decreases with use.
In place of the lead electrode associated with the above problem, an insoluble electrode coated with a platinum group metal alone or with a platinum group metal oxide is often used as the oxygen generating electrode.
This insoluble electrode usually has a structure wherein a base material, which is made of a valve metal such as titanium or an alloy containing such valve metal as a main component, is coated with an oxide catalyst layer containing, as a main component, a platinum group metal oxide such as iridium oxide or a mixture of iridium oxide and tantalum oxide. This type of insoluble electrode is scarcely consumed during electrolysis, and the oxygen overvoltage is extremely low as compared with the lead electrode.
For example, where an insoluble electrode of this type having a base material made of titanium or an alloy containing titanium as a main component and a catalyst layer made of iridium oxide is used as an anode during the manufacture of electrolytic copper foils, the plating cell voltage can be lowered by approximately 1 V, compared with the case of using the conventional lead electrode, thus making it possible to greatly cut down the power consumption during electrolysis. Further, since the electrode is scarcely consumed during the electrolytic process, the interelectrode distance remains substantially the same. Accordingly, this type of electrode serves to stabilize the processing conditions, as well as the quality of products.
Although the oxygen generating electrode having the catalyst layer containing an iridium oxide as a main component has the above-described advantages, it is still associated with a problem in that the electrolytic voltage suddenly rises after the electrolytic process is continued for a certain period of time. This phenomenon is conspicuous particularly in cases where the electrolysis is carried out at high current density. In general, when such phenomenon occurs, it is judged that the service life of the oxygen generating electrode has expired.
The above phenomenon is presumably caused by oxidation of the surface of the base material due to penetration of oxidizing substances, produced on the surface of the electrode (the surface of the catalyst layer) during electrolysis, or sulfuric acid component in the plating bath through to the surface of the base material located inward of the catalyst layer. If the surface of the base material is oxidized, the adhesion between the base material and the catalyst layer lowers or an electrical insulating oxide film is formed on the surface of the base material, lowering the electrical conductivity of the oxygen generating electrode as a whole and finally raising the electrolytic voltage.
Consequently, even though the insoluble electrode for oxygen generation shows excellent properties at an initial stage, the aforementioned phenomenon can occur, depending on the electrolysis conditions, and the service life of the electrode shortens.
To solve the problem, there have been proposed electrodes in which a primary coating consisting of a material having excellent corrosion resistance is formed on the surface of the base material prior to the formation of the catalyst layer.
For example, Examined Japanese Patent Publication (KOKOKU) No. 49-48072 discloses a method in which electrolytic or chemical oxidizing process is performed on a base material in an aqueous solution in which a valve metal, such as Ti, Ta, Nb or Zr, is dissolved, to cause an oxide of the valve metal to deposit on the surface of the base material, the resulting thin oxide layer serving as the primary coating. Then, a catalyst layer consisting of a platinum group metal or an oxide of such platinum group metal is formed on the surface of the primary coating.
The electrode produced by this method, however, has poor adhesion at interface between the primary coating and the catalyst layer. Thus, when the electrode is used as an oxygen generating electrode, the primary coating and the catalyst layer are gradually separated from each other at the interface thereof due to the effect of oxidizing substances produced on the electrode surface, and the electrode finally fails to retain its advantage of excellent durability.
Unexamined Japanese Patent Publication (KOKAI) No. 57-116786 discloses a method in which a base material is immersed in an aqueous solution in which a metal, such as Ti, Ta, Zr, Hf or Nb, is dissolved, a layer consisting of an oxide of the metal is formed on the surface of the base material by electrodeposition, as in the method disclosed in Examined Japanese Patent Publication No. 49-48072, and part of the oxide layer is subjected to heat treatment in a non-oxidizing atmosphere.
This method permits a primary coating of relatively large thickness to be formed between the base material and the catalyst layer. However, the base material and the primary coating produced by this method have poor adhesion.
Examined Japanese Patent Publication No. 60-21232 discloses a method in which a Ta compound or/and a Nb compound are thermally decomposed on the surface of a base material consisting of Ti or a Ti alloy, so that a Ti oxide present on the surface of the base material in the form of a thin film is mixed with the Ta or/and Nb oxide, thereby forming a primary coating consisting of mixed oxides.
However, the primary coating consisting of mixed oxides has poor adhesion with the base material and is also poor in corrosion resistance. Thus, the electrode produced by this method, fails to withstand long use.
In Examined Japanese Patent Publication No. 60-22074 is proposed an electrode in which a thin primary coating consisting of a mixture of Ti and/or Sn oxide and Ta and/or Nb oxide is formed on the surface of a base material by a thermal decomposition method.
In this electrode, however, although the primary coating has some degree of electrical conductivity, the corrosion resistance is poor because the primary coating is made of mixed oxides. Accordingly, the surface of the base material becomes passive with use, and a long service life cannot be expected.
In Examined Japanese Patent Publication No. 3-27635 and Unexamined Japanese Patent Publication No. 2-61083 are disclosed electrodes wherein a primary coating consisting of a mixture of an iridium oxide and an oxide of at least one substance selected from the group consisting of Ta, Ti, Nb, Sn and Zr is formed between a base material and a catalyst layer.
The primary coatings disclosed in these publications have good electrical conductivity, but since they are made of mixed oxides, the corrosion resistance is low, as in the case of the aforementioned electrodes. Thus, these electrodes do not have a long service life.
European Patent Publication No. 0538955 (A1) discloses an electrode comprising a base material made of Ti, Ta, Nb, Zr or Hf, a primary coating formed on the surface of the base material by a thermal decomposition method, and a catalyst layer formed on the primary coating and containing an iridium oxide. The primary coating is obtained by successively forming on the base material a layer of a Ti or Sn oxide, and a layer of a mixture of a Ta oxide and an Ir, Co or Pb oxide, or by successively forming on the base material a layer of a mixture of a Ta oxide and an Ir, Co or Pb oxide, and a layer of a Ti or Sn oxide.
This electrode also does not have sufficient adhesion between the base material and the Ti or Sn oxide layer, and is unable to ensure a long service life.